How can I know the viscosity of a lubricant when buying it?

The SAE (Society of Automotive Engineers) has established an internationally accepted standard based on the differentiation of viscosity at different temperatures.

This classification defines a series of grades for engine oils and contains two subclassifications: one designated by a "W" which refers to the properties of the oil at low temperatures, and another without "W" which refers to the properties at high temperatures.

Therefore, oils designated with a numerical grade followed by the letter W (such as SAE 10W) or simply with a number (such as SAE40) are called monograde oils. On the contrary, those which are designated by the two types of grade (such as SAE 10W-40) are called multigrade because they are capable of simultaneously bringing together viscosymetrical properties at both low and high temperatures.

If you want to know more about the letters on lubricant bottles, click here and discover much more.